docs: def is_mag_data(mdat):
      '''
      is_mag_data(dat) yields True if the given data is a valid set of magnification data and False
        otherwise.
  
      Note that this does not return True for all valid return values of the mag_data() function:
      specifically, if the mag_data() function yields a list of mag-data maps or a lazy-map of the
      mag-data maps split out by visual area, then this will return False. This function only returns
      True for a map of mag data itself.
      '''
      if not pimms.is_map(mdat): return False
      for k in ['surface_coordinates', 'visual_coordinates', 'mesh', 'submesh', 'mask',
                'retinotopy_data', 'masked_data', 'surface_areas', 'visual_areas']:
          if k not in mdat: return False
      return True